Site-specific phenological development compared to 2000–2011 mean of each site
Figure 2. Site-specific phenological development compared to 2000–2011 mean of each site. Grasslands are denoted by dotted and forests by striped fill patterns. Sites are ordered according to land-use type (grassland, forest) and their respective elevational gradient from left (lowest) to right (highest). The bold black line shows the mean across all sites. Negative deviations indicate earlier, positive deviations later than average phenological development in spring. Species-specific observational data were provided by MeteoSwiss for nearby sites from the national phenological monitoring network: Davos–Norway spruce (needle emergence), Laegeren–European beech and Norway spruce (leaf unfolding/needle emergence), Früebüel, Chamau, Oensingen–Cocksfoot grass and Dandelion (full flowering). Data availability for Früebüel was limited to the years 2008–2011.
Abstract
Since the European summer heat wave of 2003, considerable attention has been paid to the impacts of exceptional weather events on terrestrial ecosystems. While our understanding of the effects of summer drought on ecosystem carbon and water vapour fluxes has recently advanced, the effects of spring drought remain unclear. In Switzerland, spring 2011 (March–May) was the warmest and among the driest since the beginning of meteorological measurements. This study synthesizes Swiss FluxNet data from three grassland and two forest ecosystems to investigate the effects of this spring drought. Across all sites, spring phenological development was 11 days earlier in 2011 compared to the mean of 2000–2011. Soil moisture related reductions of gross primary productivity (GPP) were found at the lowland grassland sites, where productivity did not recover following grass cuts. In contrast, spring GPP was enhanced at the montane grassland and both forests (mixed deciduous and evergreen). Evapotranspiration (ET) was reduced in forests, which also substantially increased their water-use efficiency (WUE) during spring drought, but not in grasslands. These contrasting responses to spring drought of grasslands compared to forests reflect different adaptive strategies between vegetation types, highly relevant to biosphere–atmosphere feedbacks in the climate system.
